The present invention relates to a traction unit, particularly a rail vehicle traction unit, comprising at least one chassis bogie in which at least one geared motor unit is arranged, and relates more specifically to a traction unit drive bogie.
The bogie of a railway vehicle executes lateral movements of translation and rotational movements, known by the name of sinusoidal travel, even on a perfectly straight and flat track. The forces exerted by this sinusoidal travel on the bogie chassis and the frequencies of the resulting oscillations increase with the speed of the vehicle.
Added to these forces are the forces which are due to imperfections in the track, to jolts caused by these imperfections, to points and to curves, these forces being the cause of instability phenomena.
Because of these phenomena of instability, the speed of a vehicle fitted with such a bogie has, for safety reasons, not to exceed a speed known as the critical speed.
The speed which is known as the critical speed is the speed at which the natural frequency of the bogie is close to resonance. The frequency at which the bogie vibrates is all the higher, the higher the speed of travel of the vehicle.
It is essential, in vehicles intended to run at high speed, particularly in railway vehicles, for the so-called critical speed to be raised as high as possible by special structural arrangements so as to allow stable running.
For this reason, it is advantageous for a drive bogie to have a very low unsuspended mass so that its moment of inertia is low in comparison with the translational movement perpendicular to the direction of travel and with respect to rotations about its vertical axis.
In known traction units which are in use in very high numbers, the geared motor unit is fixed rigidly in the bogie chassis. A shaft which is preferably hollow in such cases advantageously transmits energy from the reduction gearset to the wheels via the axle.
The instability phenomena are therefore particularly sensitive as the mass of the motor and of the transmission increase the unsuspended mass of the bogie. This assembly cannot therefore be used for vehicles which have to run at high speed.
There are other rail traction units which are intended for high speeds and whose unsuspended bogie mass is reduced by fixing the motor and the transmission not to the bogie but to the chassis of the body of the vehicle.
A cardan shaft transmits energy from the reduction gearset to the wheels driven via a second gearset mounted on the axle. The lengthening and shortening of these telescopic cardan shafts introduce frictional forces which are a function of the torque that is to be transmitted and which retard the movement. Furthermore, the weight of the telescopic shaft is necessarily higher than an undivided cardan shaft.